Knee probe having increased scrub motion

ABSTRACT

Improved probing is provided using a knee probe where the knee curves away from the probe axis and then curves back to connect to the probe tip, crossing the probe axis on the way to the tip. The resulting lateral offset between the probe tip and the probe axis is a key geometrical parameter for predetermining the scrub motion provided by the probe in response to a predetermined contact force. The scrub motion preferably includes both a sliding motion and a rocking motion, where the sliding motion acts to clean the contact pad and the rocking motion acts to bring a clean part of the probe tip into contact with the freshly cleaned part of the contact pad. In preferred embodiments, the probe tip can include one or more relatively narrow “skates” for making contact to the contact pad. A dual skate configuration is especially appropriate when small dimples are at the centers of the contact pads.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No. 10/850,921, filed on May 21, 2004, entitled “Freely Deflecting Knee Probe with Controlled Scrub Motion”.

FIELD OF THE INVENTION

This invention relates to electrical probes for automated circuit and device testing.

BACKGROUND

Testing of electrical devices and circuits has been an important component of electronic manufacturing processes for some time. Such testing typically entails probing a circuit with a fixture including multiple flexible probes, each probe making electrical contact to a contact pad on the circuit chip. Various practical issues that have been noticed in this context have been addressed in the prior art.

For example, it is often necessary to probe contact pads which are closely spaced. For closely spaced contact pads, the probes may undesirably make electrical connection to each other, especially when in a deflected condition. U.S. Pat. Nos. 6,411,112 and 6,486,689 consider probe configurations for alleviating this problem.

It is well known that making electrical contact between the probe and the contact pad can be hampered by the presence of non-conductive material on the pad and/or the probe (e.g., a thin oxide film). Accordingly, considerable attention has been devoted to details of how the tip of the probe moves relative to the contact pad in order to improve the resulting electrical connection. This relative motion is usually referred to as a scrub motion. For example, U.S. Pat. No. 5,923,178 considers a probe having a shape which provides a scrub motion which is primarily a rocking motion without appreciable sliding. U.S. Pat. No. 5,952,843 considers a probe having a canted tip portion to facilitate penetration of the passivation layer. U.S. Pat. No. 6,529,021 considers a probe arrangement where the probe tip can be made to move in a reciprocating manner to reduce contact resistance.

The contact resistance issue has also been addressed by probes having separate parts for scrubbing and for making electrical contact. For example, US 2004/0239352 considers dual probes having a contact probe and a separate scrub probe, where the scrub probe moves in such a way as to clean the part of the contact pad that will end up under the contact probe during test. In some cases (e.g., copper deposition manufacturing), circuit fabrication processes provide contact pads which are covered with a protective dielectric film (e.g., a silicon dioxide film). U.S. Pat. No. 6,727,719 considers a probe having an inner contact needle and an outer hard layer, where the hard outer layer is adapted for penetrating such a protective film.

As circuit manufacturing technology continues to evolve to smaller critical dimensions, new practical issues relating to probing tend to arise which are not fully addressed by prior art approaches. For example, the decreasing size of contact pads as critical dimensions are reduced leads to increasingly demanding requirements on the ability to precisely control the probe scrub motion. Excessive scrub motion can cause loss of electrical contact, if the probe moves off the contact pad. Another consequence of device scaling is that features which are irrelevant for large probes making contact to large pads can be much more significant at a smaller scale. An example of such a feature is a dimple typically found at or near the center of a contact pad made by certain fabrication technologies (e.g., Cu contact pads on flip-chip wafers).

An important consequence of decreasing probe and contact pad dimensions is that the current density at the probe-pad contact increases. This increased current density also raises issues which have not come up before in connection with large probes on large pads. More specifically, the current density can be high enough to form micro-welds between the probe and the pad due to local heating. Breaking these micro-welds as the probe is removed from the contact pad can lead to degradation of the probe tip (e.g., by accumulation of non-conductive material), thereby reducing probe reliability and/or lifetime.

Accordingly, it would be an advance in the art to provide probing having more precise control of the scrub motion and/or addressing more specific scaling related issues(e.g., contact pad dimples and probe tip degradation from micro-welds).

SUMMARY

Improved probing is provided using a knee probe where the knee curves away from the probe axis and then curves back to connect to the probe tip, crossing the probe axis on the way to the tip. The resulting lateral offset between the probe tip and the probe axis is a key geometrical parameter for predetermining the scrub motion provided by the probe in response to a predetermined contact force. The scrub motion preferably includes both a sliding motion and a rocking motion, where the sliding motion acts to clean the contact pad and the rocking motion acts to bring a clean part of the probe tip into contact with the freshly cleaned part of the contact pad. In preferred embodiments, the probe tip can include one or more relatively narrow “skates” for making contact to the contact pad. A dual skate configuration is especially appropriate when small dimples are at the centers of the contact pads.

Embodiments of the invention provide numerous advantages. The use of a probe having an “overshoot” knee as described above and in more detail below generally tends to increase the scrub motion compared to knee probes which do not “overshoot” the probe axis. In preferred embodiments, the invention makes use of multilayer probes, which facilitates the fabrication of probes having narrow “skates” on the probe tips. Such skates advantageously decouple the contact width (which should be small to increase contact for per unit area) from the probe width (which should be large enough to prevent motion in directions other than in the intended deflection plane). Multi-layer probes also allow the skate layers to be made of a suitable tip contact material, while the remaining layers are not constrained to be suitable tip contact materials. Dual-skate probe tips can be employed to probe contact pads having dimples at their centers (e.g., as provided by plating techniques for forming contact pads). In this case, the skates advantageously avoid the dimple, thereby avoiding issues relating to degraded electrical contact and increased mechanical stress on the probe tip that can arise when probing is performed directly at dimple locations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a first contact probe configuration according to an embodiment of the invention.

FIG. 1 b shows an operational probe configuration according to an embodiment of the invention.

FIG. 2 shows a close up view of a probe tip making contact to a contact pad according to an embodiment of the invention.

FIG. 3 shows a close up view of a probe tip making contact to a contact pad according to another embodiment of the invention.

FIG. 4 shows a close up view of a probe tip making contact to a contact pad according to yet another embodiment of the invention.

FIG. 5 a shows a photograph of a probe tip.

FIG. 5 b shows a photograph of the probe tip of FIG. 5 a after 1,000,000 probing cycles according to an embodiment of the invention.

FIGS. 6 a-d are photographs of probe array configurations suitable for use with embodiments of the invention.

FIG. 7 shows a depth profile for a scrub mark made in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 a shows a “first contact” probe configuration according to an embodiment of the invention, and FIG. 1 b shows a corresponding operational probe configuration. Here “first contact” refers to the situation where a probe is in contact with a contact pad, but no contact force is applied. In contrast, an operational probe configuration makes contact with the contact pad with a predetermined contact force. Since the probe deforms in response to the contact force, the shape of the probe differs in the two cases. In particular, how the probe moves from the first contact configuration to the operational configuration is a key aspect of the invention.

A probe 100 includes a shank 102, a knee section 104 and a probe tip 106 as parts of a single structure, as shown. Shank 102 is straight and does not deflect appreciably during contact, so it is convenient to regard shank 102 as defining a probe axis 110 with which it is aligned. Knee section 104 extends from shank 102 to probe tip 106, and includes two parts. A first part of knee section 104 starts at shank 102 and extends outward from probe axis 110 to reach a knee point 112. Knee point 112 is a point of maximum separation from probe axis 110. A second part of knee section 104 starts at knee point 112 and extends to a tip location 114, such that probe axis 110 is between knee point 112 and tip location 114. A lateral tip offset 116 is thereby defined between the probe tip and the probe axis. Probe tip 106 is in contact with a contact pad 120 defining a contact point 124.

Thus knee section 104 can be regarded as extending outward for a certain distance D (the first part) and then curving back for a distance greater than D (the second part), thereby establishing the lateral offset 116. The present inventor has found that this probe configuration can provide improved probing performance. For comparison, U.S. patent application Ser. No. 10/850,921 by the present inventor considers a knee probe having a knee which curves back by a distance less than D (i.e., it does not overshoot the probe axis).

FIG. 1 b shows the corresponding operational probe configuration for the example of FIG. 1 a. Here contact pad 120 is moved toward probe shank 102 by a vertical displacement 130. Equivalently, a predetermined contact force is applied to the probe shank. For any particular probe design, there is a one to one relation (i.e., this relation is a mathematical function, which can be linear or nonlinear) between vertical displacement and contact force, as is well known in the art, so both ways of describing the operational configuration are employed interchangeably in the following description. Probe 100 deforms under the contact force, and FIG. 1 b shows key parameters of this deformation. More specifically, contact point 124′ on FIG. 1 b is farther from probe axis 110 than the corresponding contact point 124 on FIG. 1 a. Thus the probe tip slides along the contact pad for a certain distance (i.e., the difference between 116′ and 116 on FIGS. 1 a-b). In addition to this sliding motion, the probe tip also “rocks” relative to the contact pad. This rocking motion can be more clearly appreciated by defining a “tip axis” 118 on FIG. 1 a which is required to be parallel to probe axis 110 and which passes through the contact point 124. In the operational configuration of FIG. 1 b, tip axis 118 is no longer parallel to probe axis 110. The angle between tip axis 118 and probe axis 110 on FIG. 1 b is a measure of the amount of rocking motion provided.

Thus the scrub motion provided in this example includes both a sliding motion of the probe tip relative to the contact pad, and a rocking motion of the probe tip relative to the contact pad. A key aspect of the invention is that parameters of the scrub motion (e.g., slide length and rocking angle) can be predetermined, in part, by geometrical parameters of the probe and by the predetermined contact force (or equivalently, predetermined vertical displacement). More explicitly, a probing method according to the invention includes: providing a probe having the general configuration of FIG. 1 a (i.e., having a knee section with an overshoot), making contact between the probe tip and a device under test, and applying a predetermined contact force to the probe shank, thereby providing a predetermined scrub motion of the probe tip on the contact pad. The scrub motion is predetermined in part by the contact force and by geometrical parameters of the probe.

The friction provided by the contact pad is also a relevant factor for determining the scrub motion, so probe designs and/or methods will typically need to account for variations in contact pad friction. The speed with which contact is made has also been found to be relevant. More specifically, the sliding motion length on the contact pad (also referred to as scrub length) tends to decrease as the relative contact velocity between probe tip and contact pad increases. Another method of further controlling the scrub length is by laterally moving the probe as contact is made. Lateral probe motion in the direction of the tip offset will increase the scrub length, and lateral probe motion in the opposite direction will decrease the scrub length. Such lateral probe motion can be provided by appropriate motion control of a chuck holding the probe (or probes), or by appropriate motion control of a stage holding the device under test. Further scrub length control can be provided by controlling relative velocity and/or lateral probe motion. Scrub length can be measured after probing has occurred by measuring the length of the mark left by the probe on the contact pad. Such measurements are important for verifying proper probe performance.

A scrub motion including both a sliding motion and a rocking motion has provided improved results in practice. Investigations indicate that the sliding motion acts to scrape non-conductive material from the contact pad to create an exposed area of the contact pad, and the rocking motion acts to bring a clean part of the probe tip into contact with the freshly exposed area of the contact pad. From FIGS. 1 a-b, it is apparent that the rocking motion causes a different point of the probe tip to be in contact with the contact pad in the operational configuration than in the “first contact” configuration. Providing a scrub motion including both of these motions is therefore preferred.

Suitable materials for probe 100 and probe tip 106 are well known in the art, and any such materials can be employed in practicing the invention. Suitable tip materials are electrically conductive and wear-resistant, and include Rh and Cr.

Detailed design work in accordance with the above-identified principles of the invention has led to a point design as well as identification of some preferred parameter ranges. A point design for making contact to Cu or Al contact pads (or flat topped columns) has a tip offset (116 on FIG. 1 a) of 0.18 mm, a knee offset (distance between knee point 112 and probe axis 110 on FIG. 1 a) of 0.31 mm, and a probe length (combined length of probe tip and knee section in Z direction on FIG. 1 a) of 1.95 mm. In this point design, the probe width is 0.076 mm, and the probe material is Nickel-Cobalt alloy. The tip offset is preferably in a range from about 0.05 mm to about 0.25 mm. The knee offset is preferably in a range from about 0.05 mm to about 0.5 mm. The probe length is preferably between about 0.5 mm and about 3.0 mm.

For a configuration with a small knee offset and large tip offset one can expect a longer scrub length. For a configuration with large knee offset and small tip offset, a shorter scrub length is expected. Large contact friction requires a probe design that generates larger horizontal reaction force typically produced with larger scrub length. Smoother, less frictional contact pad surfaces require a probe design producing a shorter scrub length.

As indicated above, for any particular probe, there is a predetermined relation between contact force and vertical deflection. As the probe stiffness increases, the amount of contact force required for a given vertical deflection increases. A typical vertical deflection in practice is 75 μm (i.e. about 3 mils), and for this amount of deflection, the contact force is preferably between about 0.025 N and about 0.15 N (i.e., about 2.5 to 15 grams force), and is more preferably between about 0.08 N and about 0.10 N (i.e., about 8 to 10 grams force). The vertical deflection during contact is preferably between about 12 μm and about 125 μm and is more preferably between about 50 μm and about 75 μm.

Another way to describe probing according to the invention is in terms of parameters of the scrub mark left on the contact pad by the probe. The scrub mark width is preferably between about 3.0 μm and about 15.0 μm and is more preferably about 7 μm wide. The scrub mark depth is preferably between about 0.1 μm and about 2.0 μm and is more preferably about 0.6 μm. FIG. 7 shows an example of a measured scrub mark depth profile. The scrub mark length is preferably between about 3.0 μm and about 44.0 μm and is more preferably about 10 μm. This description of scrub marks assumes Al or Cu contact pads.

FIG. 2 shows a close up view of a probe tip in contact with a contact pad. More specifically, probe tip 106 makes contact with contact pad 120 on a circuit (or device) 202. Note that the view of FIG. 2 (and of FIGS. 3 and 4) differs from the view of FIGS. 1 a-b by a 90 degree rotation about the Z-axis. Thus moving left or right on FIGS. 2-4 corresponds to moving into or out of the page of FIG. 1.

FIG. 3 shows an alternative embodiment of the invention, where probe tip 106 includes multiple layers (one of which is shown as 302 and another of which is shown as 304). Such a multilayer probe configuration provides several advantages. First, as shown on FIG. 3, one of the layers (304 in this example) can extend past the others, thereby defining a “skate” having a width (i.e., y extent) substantially less than the width of probe tip 106. Reducing the probe contact area can enhance scrub motion performance, because the force per unit area is thereby increased. However, decreasing the width of the entire probe can undesirably allow the probe to deflect in the y direction. A probe tip with a skate, as shown in FIG. 3, allows most of the probe to have a y-thickness sufficient to render y-deflection negligible, while also desirably reducing the contact area.

A further advantage of the multi-layer skate configuration of FIG. 3 is that only the skate layer (i.e., layer 304) should be a material selected for suitability as a tip contacting material. The remaining layers (e.g., 302) can be selected to optimize the overall probe performance without regard for their suitability as tip materials, since they never actually make contact with contact pad 120.

FIG. 4 shows a dual-skate configuration, where probe tip 106 includes two skates 402 and 404. This dual skate configuration is suitable for probing a contact pad 120 having a dimple 406 at its center. Such a dimple is characteristic of contact pads formed by metal plating (e.g. as in flip-chip wafers). Typical dimple dimensions are about 10 μm diameter on a contact pad having a 110 μm diameter, with the size of the dimple depending on the pitch of the contact pads. A single skate configuration as in FIG. 3 will undesirably require a choice between probing at the dimple location (which can degrade the electrical contact made by the probe), or off-center probing (which can be difficult to align). Probing at the dimple can also cause high mechanical stress on the probe if the probe tip gets caught by the dimple. In contrast, the dual-skate approach of FIG. 4 avoids probing the dimple, but still has the probe tip centered on the contact, thereby simplifying automatic probe alignment.

FIGS. 5 a-b show results from an embodiment of the invention. More specifically, FIG. 5 a is a photograph of a probe tip and FIG. 5 b is a picture of the probe tip of FIG. 5 a after 1,000,000 probing cycles according to the invention. The probe of this example is a multi-layer single-skate configuration, as in FIG. 3. FIG. 5 b shows no significant degradation of the probe tip, either by wear or by accumulation of debris.

FIGS. 6 a-d are photographs of a probe array suitable for practicing the invention. Such arrays are often required in practice, since many circuits being tested have a large number of contact pads which must be probed. For probe arrays, it is important that each probe deform in a uniform and predictable manner when the contact force is applied, to prevent probe-to-probe contact resulting from probe deflection. Thus it is preferred for the probe configuration of FIGS. 1 a-b to only deform in the X-Z plane responsive to the contact force, as also indicated above in connection with probe tip skates. 

1. A method of probing a device under test, the method comprising: providing a probe including a shank defining a probe axis, a curved knee section connected to the shank, and a probe tip connected to an end of the knee section opposite from the shank, wherein a first part of the knee section extends outward from the probe axis and reaches a knee point of maximum separation from the probe axis, thereby defining a lateral knee offset, wherein a second part of the knee section extends from the knee point to a tip location such that the probe axis is between the tip location and the knee point, thereby defining a lateral tip offset from the probe axis; making contact between the probe tip and a contact pad of the device under test; and applying a predetermined contact force to the probe shank, whereby a predetermined scrub motion of the probe tip on the contact pad is provided; wherein the scrub motion is predetermined in part by the contact force and by geometrical parameters of the knee section.
 2. The method of claim 1, further comprising decreasing a length of the predetermined scrub motion on the contact pad by increasing a relative contact velocity between the contact pad and the probe tip.
 3. The method of claim 1, further comprising altering a length of the predetermined scrub motion on the contact pad by laterally moving the probe with respect to the contact pad as contact is made.
 4. The method of claim 1, wherein the predetermined scrub motion includes a sliding motion of the probe tip relative to the contact pad and a rocking motion of the probe tip relative to the contact pad.
 5. The method of claim 4, wherein the sliding motion acts to scrape non-conductive material from the contact pad to create an exposed area of the contact pad, and wherein the rocking motion acts to bring a clean part of the probe tip into contact with the exposed area.
 6. The method of claim 1, wherein the shank and knee section have a probe width in a direction perpendicular to the probe axis and perpendicular to the lateral knee offset, wherein the probe tip includes a first skate section having a skate width in said direction substantially less than the probe width, and wherein the first skate section of the probe tip faces away from the shank and is capable of making contact with the contact pad.
 7. The method of claim 6, wherein the probe comprises multiple layers stacked in said direction, and wherein the first skate section includes only one of the layers.
 8. The method of claim 6, wherein the probe comprises multiple layers stacked in said direction, wherein said first skate section includes only one of the layers, and wherein the probe tip further comprises one or more additional skate sections, each of the additional skate sections including only one of the layers.
 9. The method of claim 1, wherein a combined length of the probe tip and the knee section is between about 0.5 mm and about 3.0 mm.
 10. The method of claim 1, wherein the lateral knee offset is between about 0.05 mm and about 0.5 mm.
 11. The method of claim 1, wherein the lateral tip offset is between about 0.05 mm and about 0.25 mm.
 12. The method of claim 1, wherein the predetermined scrub motion provides a scrub mark having a width between about 3.0 μm and about 15.0 μm, and wherein said contact pad comprises Al or Cu.
 13. The method of claim 1, wherein the predetermined scrub motion provides a scrub mark having a depth between about 0.1 μm and about 2.0 μm, and wherein said contact pad comprises Al or Cu.
 14. The method of claim 1, wherein the predetermined scrub motion provides a scrub mark having a length between about 3.0 μm and about 44.0 μm, and wherein said contact pad comprises Al or Cu.
 15. The method of claim 1, wherein the predetermined contact force is between about 0.025 N and about 0.15 N for 75 μm vertical deflection of the probe tip.
 16. The method of claim 15, wherein the predetermined contact force is between about 0.08 N and about 0.10 N at 75 μm vertical deflection of the probe tip.
 17. The method of claim 1, wherein a vertical deflection of the probe tip during contact is between about 12 μm and about 125 μm.
 18. The method of claim 17, wherein the vertical deflection of the probe tip during contact is between about 50 μm and about 75 μm. 